Recently, with practical implementation of a blue light-emitting diode (LED), development of a white LED using this blue LED is being aggressively pursued. Use of a white LED in liquid crystal panel backlight, indoor or outdoor lighting device, etc. is evolving and expanding because of its low power consumption and long life compared with existing white light sources.
The white LED having developed at present is obtained by applying a Ce-doped YAG (yttrium-aluminum-garnet) onto the surface of a blue LED. The Ce-doped YAG phosphor exhibits a fluorescence spectrum having a fluorescence peak wavelength in the vicinity of 560 nm and a dominant wavelength in the vicinity of 570 nm. However, the YAG phosphor activated by Ce has a problem in that the fluorescence intensity deteriorates as the temperature increases, namely, the temperature characteristics are poor.
Considering the above, many oxynitride phosphors are being studied and among others, an α-SiAlON phosphor activated by Eu is known to have excellent temperature characteristics and emit fluorescence having a fluorescence peak wavelength (yellow-orange) of 580 to 590 nm (see, Patent Document 1).
Other than the α-SiAlON activated by Eu, a Ca-α-SiAlON activated by Ce is known to emit blue to green fluorescence (see, Patent Documents 2 and 3). In addition, a Ca-α-SiAlON activated by Pr is known to have a fluorescence peak at 450 to 750 nm (see, Patent Document 4). Furthermore, it is known that a Ca-α-SiAlON activated by Yb exhibits green emission having a fluorescence peak of about 550 nm (see, Patent Document 5) and a Ca-α-SiAlON co-activated by Eu and Dy exhibits yellow emission having a fluorescence peak wavelength of about 580 nm (see, Patent Document 4).
However, the Ce-activated Ca-α-SiAlON described in Patent Documents 2 and 3 shows a very low efficiency by excitation with excitation light of 450 nm, and the Pr-activated Ca-α-SiAlON described in Patent Document 4 does not have an excitation band in the vicinity of 450 nm, making it difficult to obtain high fluorescence efficiency by using a blue LED.
As described above, specific examples of an α-SiAlON phosphor having yellow emission comparable to that of a YAG phosphor activated by Ce have not been disclosed.